Light emitting diodes have emerged in recent years as viable sources of light. Light emitting diodes, also called solid-state lighting devices or simply LEDs, are highly efficient, durable and long lasting lighting devices. The technology has improved enormously since the 1960s when the first LEDs came to market. LEDs are now the industry standard in a variety of specialty lighting markets and the popular bulbs are rapidly entering the general illumination market. LED bulbs are more energy efficient and last longer than, for example, incandescent, halogen and fluorescent bulbs. Advances in technology have provided LEDs that are typically four to five times more efficient than incandescent bulbs and have lifetimes exceeding tens of thousands of hours.
LEDs are current-driven devices, whose brightness is proportional to their average forward current (also referred to as their average load current). For this reason LEDs are usually driven using a current source that provides a constant current. The constant-current source eliminates load current variations resulting from variations in the forward voltage of a LED and thus ensures a constant LED brightness. In known LED drivers, which are often implemented as switching converters such as buck, boost, or buck-boost converters, a plurality of components are integrated that evaluate voltages and compare those voltages to a reference voltage. Usually a power semiconductor switch (e.g., a MOSFET) is switched on and off in accordance with the results of this comparison, in order to charge or discharge an inductor.
The application note AN874, “Buck Configuration High-Power LED Driver,” Microchip Technology, 2006, describes a switching power supply circuit that controls the load current supplied to an LED. However, during the delay time period that is needed to perform the measurements of the LED current and to activate the switch (e.g., due to propagation delays), in order to charge or discharge the inductor, the desired maximum value of the LED current is exceeded. This results in a mismatch between the desired average load current and the actual average load current supplied to the LED, which results in an undesired increase of brightness of the LEDs.
Although this mismatch may be considered during circuit design, the average load current supplied to the LED and thus the LED brightness itself will be different for different forward voltages (which are temperature-dependent) of the LED as well as for a different number of LEDs connected in series and for different supply voltages applied to the LED and the LED driver. That is, common LED drivers—even when designed as current sources—are usually not able to keep the average load current constant (e.g., while the supply voltage or the LED forward voltages are varying for different inductance values of the inductor) due to the delay time periods mentioned above. Thus, the LED drivers have to be reconfigured for each different situation.
A cost-efficient, but effective solution is needed that ensures (an almost) constant brightness for different supply voltages or different LED forward voltages without the need of reconfiguring the circuit.